An old (~2021) yuqk change for which I could not find a prior newsgroup 
post 'description', allows negative phong and spectral values.

We have always been able to use the finish{} block phong and specular 
features together for effects in an additive-color way. The yuqk fork 
allows negative phong/spectral values too. For example:

     #declare PirateGold = srgb <0.72941,0.49804,0.01176>;
     #declare Fin01 = finish {
         brilliance 2.2
         diffuse    0.6
         metallic   1.0
         phong     -3.3 phong_size 47    // <---
         specular   3.3 roughness  1/20
         intensity_max 1.0
     }
     #declare Txtr00 = texture {
         pigment { PirateGold }
         normal { granite bump_size 0.5 scale 1/2.5 }
         finish { Fin01 }
     }

This capability, along with the previously discussed, new to yuqk 
feature, intensity_max, allows for highlights which end in white while 
staying within zero to one rgb channel value ranges(*).

---

In the attached image matrix the columns left to right show: phong only, 
spectral only and finally combined phong and spectral use on the right.

The top row shows conventional double use of phong and spectral features 
where, especially together, the output result is clipped to a strong 
yellow (the hue is shifting(**)). If the light source were much 
stronger, that strong yellow would eventually go white in the most 
intense phong/specular regions(**).

The bottom row uses new yuqk finish features as shown in the code above. 
  The differences being the intensity_max option better maintains the 
phong/spectral metallic hue throughout. Coupled with a tighter phong 
highlight and use of a negative phong value, this results in overall 
highlights which gradually move from PirateGold to white within a zero 
to one rgb channel value range (lower right).

Bill P.

(*) - Zero to one rgb channel ranges work better / more cleanly with 
antialiasing (AA) - and many other POV-Ray features.

(**) - Where the output image channel ranges are clipped by the image 
format and/or the display monitor's capabilities.

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